This invention realtes to laser circuitry, and more particularly to a laser driver circuit for modulating the light output of the laser between high and low light output levels in response to an input digital signal and which maintains the low light output level of a laser at the same fixed set level regardless of laser to laser variations and regardless of changes in laser characteristics due to aging and temperature effects.
In order to transmit digital signals over optical facilities, lasers are typically used to convert input electrical signals into modulated light waves. By modulating the current through the laser by the digital electrical signal, the light output of the laser is modulated between a low output power level, L.sub.0, and a high output power level, L.sub.1, in accordance with the ONEs and ZEROes of the input digital signal. This light modulated signal when coupled to an optical fiber for transmission, can be detected by a receiver which can convert the light signal back to an electrical signal by distinguishing the ONEs and ZEROes in accordance with the varying intensity of the received optical signal. It is generally desirable to bias the laser for the low output light level with a current, I.sub.b, that is as close to the laser threshold point as possible, the threshold being the point at which the laser commences to produce a stimulated light emission. The laser is then modulated with a modulating current, I.sub.m, that has a magnitude so that the extinction ratio, .gamma., defined as the ratio of L.sub.1 /L.sub.0, is large enough to enable the receiver to accurately distinguish between a received ONE and ZERO. If the laser is biased too far below threshold, the light output will not be linearly related to the magnitude of modulating current which will degrade receiver error performance due to the reduction and uncertainty in the level L.sub.1. In addition, a bias too far below threshold will degrade the rise time and shape of the transition between levels L.sub.0 and L.sub.1. Similarly, a bias too high above threshold does not fully utilize the linear dynamic range of the laser and will also degrade receiver error performance due to decreased receiver sensitivity to a too high low light output level. Accordingly, it is desirable to bias a laser as close to threshold as possible. Whereas lasers had most often in the past been biased just below threshold, it has recently been found (see for example, S. E. Miller, "Turn-on Jitter in Nearly Single-Mode Injection Lasers," IEEE Journal of Quantum Electronics, Vol QE-22, No. 1, p. 16, January 1986,) that turn-on delay and turn-on jitter can degrade performance and limit the maximum speed at which the laser can be operated. Thus, bias just above the threshold is now most perferable for high speed operation.
One cannot bias a laser with a fixed current, however, since many factors affect laser performance. Particularly, a laser is sensitive to changes in temperature, aging and the data modulated thereon. Furthermore, as the laser is operated, its characteristics may vary. These changing characteristics will affect the laser threshold and the laser efficiency, .differential.L/.differential.I, the slope of the light output power-input current curve. In addition, the characteristics of a laser will vary from device-to-device for a given type of laser. As a result, a bias current selected at one instant for one device, may fall way above or below threshold on another device, or as the laser is operated, thereby seriously affecting performance.
Various prior art techniques and circuits have been devised to stabilize the bias level of a laser. An all electronic circuit is described in U.S. Pat. No. 4,081,670 to A. Albanese, issued Mar. 28, 1978. In this circuit, changes in the junction voltage of a laser are monitored to determine when the laser falls below threshold and to set the bias current. The circuit cannot be used to bias the laser above threshold since the information needed to adjust the bias point would not be present above threshold.
More typical arrangements involve the use of an optical feedback circuit to stabilize the laser. One common circuit is illustrated in FIG. 157 of the text, Optical Fibre Communication Systems, Edited by C. P. Sandbank, John Wiley & Sons, 1980, p. 210. In this system the optical power emanating from the back-facet of the laser is measured by a slow photodetector and the mean output power of the photodetector is adjusted to be a constant. A long string of ZEROes or ONEs in the data pattern, however, will cause the bias level to be adjusted up or down to maintain the constant average measured power. Also, there is no compensation for variations in slope efficiency of the laser and each laser must be individually tuned.
More sophisticated optical feedback circuits estimate the low output power level, L.sub.0, to determine the bias current that should be supplied to the laser. One such laser bias control circuit that compensates for changes in threshold due to temperature variations is shown in FIG. 5.20 of the text, Topics in Applied Physics, Semiconductor Devices for Optical Communication, Volume 39, Second Updated Edition, Springer-Verlag, New York, 1982, p. 184. In this circuit the bias is adjusted by the feedback loop so that the average photocurrent generated by a photodetector is held constant with respect to the average of the input driving voltage data pattern. By comparing the data pattern with the photodetector output, control is maintained independent of the duty cycle of the data. Accurate adjustment in the feedback loop is, however, dependent in part upon the coupling coefficient of the photodetector which is responsive to the light output of the laser and upon the laser efficiency, .differential.L/.differential.I. This photodetector coupling coefficient, .eta., varies from device to device, and thus this circuit cannot automatically compensate for changes due to device parametric variations. Thus each such circuit must be individually tuned, which is highly labor intensive and disadvantageous for mass production of laser devices for consumer applications. In addition, the circuit does not compensate for changes in slope efficiency.
Another optical feedback circuit is described in "Laser Level-Control Circuit for High-Bit-Rate Systems Using a Slop Detector", Electronics Letters, Vol. 14, No. 24, pp. 775-776, Nov. 23, 1978. In this circuit a low frequency small amplitude signal is superimposed on the laser current when the data is ZERO. The low frequency modulation of the laser is detected by a slow feedback photodiode, the amplitude of the detected low frequency signal being indicative if the L.sub.0 light output level is above or below threshold. The mean power is also monitored by the same photodiode so that the L.sub.1 output power level can be determined and controlled. This circuit is more complicated than the previously described circuit and does not permit bias above threshold.
A circuit that does not require device-to-device adjustments and does compensate for changes in threshold and slope efficiency due to temperature variations and aging is shown in FIG. 161 of the aforenoted text, Optical Fibre Communication Systems, p. 213. In this circuit the monitored optical waveform is sampled in both the ONE level and ZERO level and compared with preset demanded values and the bias current and modulation current adjusted accordingly. This circuit requires two feedback loops, however, therefore requiring a more complex circuit realization that is difficult to implement at high speeds.
In general, the less complicated prior art circuits provide inadequate allowance for device parametric variations and require stringent manufacturing control to achieve desired performance. Also, the less complicated prior art circuits do not provide correction for changes in slope efficiency. The more complicated prior art circuits that do provide sufficient compensation, have satisfactory performance only at low data rates.
A simple laser drive circuit for modulating the output power of a laser in response to an input digital signal and that maintains the operating point of the laser at a relatively stable point regardless of temperature, aging, and device-to-device variations is desirable for high speed optoelectronic circuits.